In wireless communication networks, it is know to assign resource elements of the available radio capacity to be used for data transmission to or from a user equipment (UE). Depending on the underlying radio access technology, such resource elements may correspond to time slots and/or to portions of the available frequency spectrum.
For example, a radio access technology specified by 3GPP (3rd Generation Partnership Project) and referred to as LTE (Long Term Evolution) uses Orthogonal Frequency Division Multiplexing (OFDM) for downlink (DL) transmissions to UEs and Discrete Fourier Transform (DFT) spread OFDM for uplink (UL) transmissions from the UEs. In this case, the available resources may be organized in a time-frequency grid of subcarriers with 15 kHz width and time slots corresponding to the duration of one OFDM symbol. A resource element may then extend over one subcarrier in the frequency domain and the duration of one OFDM symbol in the time domain. Such a time-frequency grid may be defined individually for each antenna port. In this connection, an antenna port may be defined such that the channel over which a certain OFDM symbol on the antenna port is conveyed can be inferred from the channel over which another symbol on the same antenna port is conveyed.
In the time domain, LTE DL transmissions are organized in radio frames of 10 ms duration, each radio frame consisting of ten equally-sized subframes of 1 ms duration. The subframes are in turn divided into two slots, each having 0.5 ms duration. Each subframe includes a number of OFDM symbols which may be used for conveying control information or data.
The resource allocation in LTE is defined in terms of resource blocks. A resource block corresponds to one time slot in the time domain, i.e., having the duration of one OFDM symbol, and 12 contiguous subcarriers in the frequency domain. In LTE, the highest granularity level of assigning resource elements corresponds to two in time consecutive resource blocks, also referred to as a resource block pair. Dynamic scheduling may be performed in each subframe. For this purpose, an LTE base station referred to as eNB may use a DL control channel, termed as Physical DL Control Channel (PDCCH) or enhanced PDCCH (ePDCCH) to transmit DL assignments and UL grants to the UEs served by the base station. Such DL control channels are transmitted in the first OFDM symbol(s) of the subframe and typically spans substantially the whole system bandwidth, i.e., all available subcarriers of the utilized carrier. If an UE has decoded such DL assignment, it knows which time and frequency resources in the subframe contain DL data destined to the UE. Similarly, upon receiving an UL grant, the UE knows on which time/frequency resources it should transmit UL data. The DL data are carried by a channel which is shared by the UEs served by the base station and is referred to as Physical DL Shared Channel (PDSCH). Similarly, the UL data are carried by a channel which is shared by the UEs served by the base station and is referred to as Physical UL Shared Channel (PUSCH).
The ePDCCH is generally similar to the PDCCH, but offers some enhanced functionalities and may be subject to additional requirements. For example, while the PDCCH requires cell specific reference symbols (CRS) for its demodulation, the ePDCCH may require UE specific demodulation reference symbols (DMRS) for its demodulation. This may allow for utilizing UE specific spatial processing for transmission of the ePDCCH.
In the above-mentioned LTE radio access technology, but also in other radio access technologies, demodulation and decoding of sent data typically requires estimation of a propagation characteristic of the radio channel. This may be accomplished by using transmitted reference symbols (RS), i.e., symbols known by the receiver. In LTE, CRS are transmitted in all DL subframes. Besides their usage for DL channel estimation, they may also be used for mobility measurements performed by the UEs. In addition, also UE specific RS (also referred to as DMRS) may be used. The UE specific RS are typically dedicated exclusively to radio channel estimation for demodulation purposes. In LTE, the UE specific RS are precoded along with the data transmitted to the UE. Accordingly, the radio channel characteristic estimated from the UE specific RS also includes the precoding operation. Accordingly, the precoding operation may become transparent to the UE, which means that the radio channel characteristic estimated from the UE specific RS can be directly applied by the UE for spatial filtering or demodulation of the received data, without explicitly taking into account the precoding operation.
The above-mentioned transparency of the precoding operation to the UE it allows for flexibly adapting precoding and/or beamforming when performing DL transmissions to the UE. However, channel estimation filtering using permutations in time and/or frequency may be constrained. For example, the effective radio channel traversed by the UE specific RS can change abruptly when the transmitter updates the precoding with respect to its time and/or frequency characteristics. In view of this situation, a concept referred to as precoding resource block (PRB) bundling was introduced in LTE. In these concepts, a group of frequency consecutive PRB pairs within a subframe are grouped into a precoding resource group (PRG). For such PRG, the UE can assume that the precoding operation remains static. This ensures that that the UE can effectively perform time and frequency domain filtering throughout the PRG so as to accomplish estimation of the propagation characteristic of the radio channel. The size of the PRG may range from one to three PRBs, depending on the system bandwidth.
The above concepts of PRB bundling tend to limit the performance of radio channel estimation and hence demodulation performance. Further, signal overhead due to the UE specific RS may need to be relatively high to allow sufficient radio channel estimation accuracy.
Accordingly, there is a need for techniques which allow for efficiently utilizing precoded RS in data transmission.